The present invention pertains generally to ophthalmic laser surgery procedures. More particularly, the present invention pertains to laser surgical procedures which are performed to reshape or restructure the cornea of an eye by using photodisruption techniques to remove stromal tissue. The present invention is particularly, but not exclusively, useful as a method and system for focusing laser energy inside a stromal lamella to photodisrupt stromal tissue.
It is well known that the refractive properties of the cornea can be altered by the selective removal of corneal tissue. For example, a myopic condition of the eye can be corrected by selectively removing corneal tissue from the central portion of the cornea. Similarly, a hyperoptic condition can be corrected by selectively removing corneal tissue within a peripheral ring surrounding the central portion of the cornea.
A general knowledge of the anatomy of the cornea of an eye is helpful for appreciating the problems that must be confronted during refractive corrections of the cornea. In detail, the cornea comprises various layers of tissue which are structurally distinct. In order, going in a posterior direction from outside the eye toward the inside of the eye, the various layers in a cornea are: an epithelial layer, Bowman""s membrane, the stroma, Decimet""s membrane, and an endothelial layer. Of these various structures, the stroma is the most extensive and is generally around four hundred microns thick. Additionally, the healing response of the stromal tissue is generally quicker than the other corneal layers. For these reasons, stromal tissue is generally selected for removal in refractive correction procedures.
In detail, the stroma of the eye is comprised of around two hundred identifiable and distinguishable layers of lamellae. Each of these layers of lamellae in the stroma is generally dome-shaped, like the cornea itself, and they each extend across a circular area having a diameter of approximately six millimeters. Unlike the layer that a particular lamella is in, each lamella in the layer extends through a shorter distance of only about one tenth of a millimeter (0.1 mm) to one and one half millimeters (1.5 mm). Thus, each layer includes several lamellae. Importantly, each lamella includes many fibrils which, within the lamella, are substantially parallel to each other. The fibrils in one lamella, however, are not generally parallel to the fibrils in other lamellae. This is so between lamellae in the same layer, as well as between lamellae in different layers. Finally, it is to be noted that, in a direction perpendicular to the layer, each individual lamella is only about two microns thick.
Within the general structure described above, there are at least three important factors concerning the stroma that are of interest insofar as the photodisruption of stromal tissue to effect a refractive change is concerned. The first of these factors is structural, and it is of interest here because there is a significant anisotropy in the stroma. Specifically, the strength of tissue within a lamella is approximately fifty times the strength that is provided by the adhesive tissue that holds the layers of lamellae together. Thus, much less energy is required to separate one layer of lamellae from another layer (i.e. peel them apart), than is required to cut through a lamella. The second factor is somewhat related to the first, and involves the response of the stromal tissue to photodisruption. Specifically, for a given energy level in a photodisruptive laser beam, the bubble that is created by photodisruption in the stronger lamella tissue will be noticeably smaller than a bubble created between layers of lamellae. This is important because the creation of large bubbles tends to cloud the cornea, and thereby reducing the effectiveness of wavefront analysis during the procedure. Additionally, at a given laser energy, much more tissue is photodisrupted when the laser beam is focused inside a lamella than when the laser beam is focused between layers of lamellae.
After the photodisruption of stromal tissue, water resorption occurs, lessening the effect of the photodisruption. For some tissues, up to 80% of the water vapor produced by photodisruption is resorbed. Thus, the present invention recognizes that photodisruption is more effective on some types of stromal tissue than others. It is also preferable to create small bubbles inside the stromal lamellae to effect a refractive change in the cornea by photodisruption. The third factor concerning the stroma is optical, and it is of interest here because there is a change in the refractive index of the stroma between successive layers of lamellae. This is due to differences in the orientations of fibrils in the respective lamella.
Somewhat related to the present invention, a method for finding an interface between layers of lamellae for photodisrupting using a wavefront analyzer and an ellipsometer was disclosed in co-pending U.S. patent application Ser. No. 09/783,665, filed on Feb. 14, 2001 by Bille and entitled xe2x80x9cA Method for Separating Lamellaexe2x80x9d. As such, the contents of co-pending application Ser. No. 09/783,665 are hereby incorporated herein by reference. In co-pending application Ser. No. 09/783,665, a procedure for creating a corneal flap for a LASIK type procedure was presented. Unlike the present invention, the goal in the creation of a corneal flap is to minimize the total amount of tissue that is photodisrupted while establishing a continuous cut of stromal tissue.
In light of the above, it is an object of the present invention to provide a device and method for positioning the focal point of a laser beam inside a stromal lamellae and maintaining the focal point at locations inside the stromal lamellae to photodisrupt stromal tissue and alter the refractive properties of the eye. Another object of the present invention is to provide a method for using a laser beam to photodisrupt relatively large amounts of stromal tissue with a laser beam of relatively low energy. Still another object of the present invention is to provide a method for photodisruption of stromal tissue that avoids the large bubbles and associated clouding that occurs when the laser beam is focused on tissue lying on an interface between layers of lamellae. Another object of the present invention is to provide a device and method for tracking the progress of the photodisruption procedure, providing information that can be used to update the amounts and locations of stromal tissue that must be removed to obtain the desired refractive correction. Yet another object of the present invention is to provide a method for altering the refractive properties of the cornea that is easy to perform and is comparatively cost effective.
In accordance with the present invention, a method for altering the refractive properties of the cornea involves photodisrupting tissue at selected locations within the stroma of the cornea. Specifically, each photodisruption location is selected to preferably be inside a stromal lamella rather than at a location between lamellae. By photodisrupting a plurality of stromal lamellae in this manner, the refractive properties of the cornea can be altered at relatively low laser energies and with minimal clouding of the cornea. To photodisrupt a location inside a stromal lamella, the focal point of the laser, and consequently the laser energy, is focused inside a stromal lamella.
For the present invention, a wavefront detector can be used during the photodisruption procedure to track the progress of the corrective procedure. Using the wavefront detector, continuously updated information concerning the refractive properties of the cornea is provided to the surgeon during the course of the procedure. This continually changing information allows the surgeon to select the amounts and locations of stromal tissue that must be subsequently altered to obtain the desired shape for the cornea.
To locate the focal point inside a stromal lamella in accordance with the present invention, the laser beam is initially focused to a start point in the stroma at a depth of approximately one hundred and eighty microns from the anterior surface of the cornea. As contemplated by the present invention, the anterior surface of the cornea can be identified using a wavefront detector. Preferably, the laser beam is set to operate at an energy level that is slightly above the threshold for photodisruption of stromal tissue (i.e. slightly above approximately five microjoules for a ten micron diameter spot size). For example, the initial energy level used for the laser beam may be around six microjoules for a ten micron diameter spot.
Once the start point is located, tissue at the start point is photodisrupted by the laser beam to generate a photodisruptive response (i.e. a bubble is created). The size of this bubble is then measured and compared with a reference value to determine whether the bubble was created inside a lamella or between layers of lamellae. This measurement of the bubble is preferably accomplished with a wavefront detector. If it is determined that the initial bubble was created between layers of lamellae, a subsequent bubble is created at a different point in the stroma. In most cases, this subsequent bubble is created at a smaller depth from the anterior surface of the cornea than the initial bubble. The new bubble is then compared to the reference value to determine whether the new bubble was created inside a lamella. This process is continued until a bubble is created having a bubble size indicating that photodisruption is occurring inside a lamella.
For the purposes of the present invention, the reference value is chosen to correspond to a hypothetical gas bubble in the stroma that, as a result of photodisruption, would have a diameter of approximately fifteen microns. A condition wherein the measured bubble is less than the reference value is indicative that the photodisruption of tissue is occurring in the stronger tissue that is located on the inside of a lamella, rather than at an interface between layers of lamellae. Accordingly, further photodisruption is accomplished by maintaining the initial depth of the laser and moving its focal point to create the desired photodisruption pattern at locations inside the lamella. On the other hand, when the measured bubble is greater than the reference value, the indication is that the focal point is no longer located inside a lamella. Thus, in accordance with the present invention, the depth of the focal point is altered until the subsequent photodisruption occurs inside a lamella (i.e. until a bubble is produced that is smaller than the reference value).
Once a bubble is created indicating that photodisruption has occurred at a location inside a lamella, an ellipsometer can be used to detect a birefringent condition at the location. Specifically, this birefringent condition results from the orientation of fibrils in the lamella. Further, it is known that from layer to layer of lamellae there will be a birefringent change that is manifested as a change in phase of about one half degree. In accordance with the present invention, the detection of the birefringent change can indicate a change from one layer of lamellae to another. Consequently, detection of the birefringent change can be used to establish and maintain a desired focal depth in the stroma.